I'm pretty sure I've screwed myself out of some good career opportunities because I was too afraid of failing at them and, in my mind, "forever ruining my reputation."
@Elegy A little of both. There were a few high-importance projects that were offered to me that, if successful, might have led to an interesting different career path, but if it failed, would mean a project that hundreds of people worked to get off the ground would go no farther. I just couldn't handle that weight on my shoulders, even though its fate would ultimately be out of everyone's hands.